In thermal dye transfer printing, an image is formed on a receptor sheet by selectively transferring a dye to a receptor sheet from a dye donor sheet placed in momentary contact with the receptor sheet. Material to be transferred from the dye donor sheet is directed by a thermal printhead, which consists of small electrically heated elements (print heads). These elements transfer image-forming material from the dye donor sheet to areas of the dye receptor sheet in an image-wise manner. Thermal dye transfer systems have advantages over other thermal transfer systems, such as chemical reaction systems, and thermal mass transfer systems. In general thermal dye transfer systems offer greater control of gray scale than these other systems, but they have problems as well. One problem is release of the dye donor and receptor layers during printing. This has been addressed often by the addition of dye-permeable release coatings applied to the surface of the dye receptor layer. Additionally, materials are required for use in the receptor layer having suitable dye permeability, mordanting properties, adhesion to the substrate, and long term light and thermal stability.
It is often desirable for imaging systems to be useful on different types of receptor substrates. This is particularly true in imaging systems used in proofing operations where the final image is to give the appearance of the final printed product. Where the final product is to be an image provided on a metal surface, it is difficult for many types of proofing systems to provide a realistic imaging system. Conventional photomechanical proofing systems which operate by using specialized photoresist or phototackifiable elements can not readily provide quality images on metal surfaces as they are not designed for such image productions.
Thermal dye transfer and thermal mass transfer often require special receptor surfaces to provide the highest quality images. Thermal mass transfer has been able to provide fairly accurate renditions of images on metallized surfaces by first transfering a metallic thermal mass transfer background and then transfering a colored image on top of or within the borders of the metallic image. Thermal dye transfer can not perform that type of two step image transfer because metallic appearing images or backgrounds are not readily provided by dyes or dye-like materials susceptible to the thermal dye transfer process. Recent advances in thermal dye transfer imaging systems describe thermal dye transfer receptor layers which can be coated out of solution and used on various surfaces to provide a high quality dye receiving layer. Such materials are described in U.S. Pat. Nos. 4,914,078 and 4,968,658. These layers are coated out of solution from presently acceptable solvents such as methyl ethyl ketone. These types of coating solutions were, however, not found to be useful on commercially available metallized (e.g., aluminized) polyester film. The most serious problem encountered in the use of these specialized dye receptive coatings was that the coating would not strongly adhere to the metallized surface. Even dye receptive coatings that were known to have good adherent properties to metal were found to suffer from poor adhesion to the metallized surface. It was discovered that the deposition of the vaporized metal onto the polymer surface caused the underlying polymer on the surface to decompose, usually into dimers, trimers and other lower molecular weight materials that would collect onto the surface of the metallized layer. These materials would act much like a release surface and greatly weaken the adherence of coatings applied thereto. Additionally, these materials would not remove easily when treated by washing with ketones, alcohols, acetones, tetrahydrofuran, or toluene. In fact, some of these solvents were not able to easily penetrate the coating formed by the decomposition product of the polymer which deposited itself on the surface of the vapor deposited metal.
Polyvinyl chloride derivatives and copolymers have been heavily used in thermal dye transfer receptor sheets as receptor layers, because of their properties in these areas. For example, U.S. Pat. No. 4,853,365 discloses that chlorinated polyvinyl chloride, used as a dye receptor, has good dye solubility and high dye receptivity. Similarly, vinyl chloride/vinyl acetate copolymers have also been used as receptor layers in thermal dye transfer receptor sheets as described in Japanese published application nos. 29,391 (1990) and 39,995 (1990). Japanese published application no. 160,681 (1989) discloses dye acceptance layers comprising polyvinyl chloride-polyvinyl alcohol copolymers, and Japanese published application nos. 43,092 (1990), 95,891 (1990) and 108,591 (1990) discloses dye image receiving layers comprising a hydroxy modified polyvinyl chloride resin and an isocyanate compound. U.S. Pat. No. 4,897,377 discloses a thermal transfer printing receiver sheet comprising a supporting substrate coated on at least one surface with an amorphous polyester resin. Published European patent application 133,012 (1985) discloses a heat transferable sheet having a substrate and an image-receiving layer thereon comprising a resin having an ester, urethane, amide, urea, or highly polar linkage, and a dye-releasing agent, such as a silicone oil, being present either in the image-receiving layer or as a release layer on at least part of the image receiving layer. Published European patent application 133,011 (1985) discloses a heat transferable sheet based on imaging layer materials comprising first and second regions respectively comprising (a) a synthetic resin having a glass transition temperature of from -100.degree. to 20.degree. C., and having a polar group, and (b) a synthetic resin having a glass transition temperature of 40.degree. C. or above.
U.S. Pat. No. 4,968,658 teaches the use of a thermal transfer receptor surface with a receiver coating comprising a dye-receptive material, a dye-permeable release agent, and an alkoxylated Bisphenol A unsaturated polyester. Fumaric acid may also be used in the layer.